High-frequency heating devices comprising a waveguide for heating thin widths of material



1968 F. TIMMERMANS ETAL 3,413,433

HIGH-FREQUENCY HEATING DEVICES COMPRISING A WAVEGUIDE FOR HEATING THIN WIDTHS OF MATERIAL Filed March 28, 1966 2 Sheets-Sheet l v v v r r v vr INVENTORS FRANCISCUS TIMMERMANS WERNER GOLOMBEK BY lama E AGENT N 1968 F TIMMERMANS ETAL HIGH-FREQUENCY HEATING DEVICES COMPRISING A WAVEGUIDE FOR HEATING THIN WIDTHS OF MATERIAL Filed March 28, 1966 2 Sheets-Sheet 2 FIGA FIGZ

AR(cm) a (cm) FIGS INVENTORSS FRANGISCUS TIMMERMANS BY WERNER GOLOMBEK AGE United States Patent 3,413,433 HIGH-FREQUENCY HEATING DEVICES COM- PRlSlNG A WAVEGUIDE FOR HEATING THEN WIDTHS 0F MATERIAL Franciscus Timmermans, Harksheide, and Werner Golomhek, Quickborn, Germany, assignors to North American Philips Company, inc., New York, N.Y., a corporation of Delaware Filed Mar. 28, 1966, Ser. No. 537,727 Claims priority, application Germany, Mar. 27, 1965, P 36,396 17 Claims. (Cl. 219-1061) ABSTRACT OF THE DISCLOSURE A high frequency heating device for heating strip dielectric material comprises a folded waveguide having a continuous longitudinal slot through which the strip material is passed. High frequency energy is propagated through the waveguide. The wavelength in the waveguide is varied during the heating operation by effectively varying the cross-sectional dimensions of the waveguide, e.g., by means of a driving rod attached to the top of the waveguide and reciprocally driven vertically by a cam arrangement.

This invention relates to high-frequency heating devices for heating thin strips of material. More particularly, to a high frequency heating device comprising a waveguide provided at the periphery with two longitudinal slots over at least part of its length, the strip of material being arranged in the plane of connection of the slots (plane of the slots) which extends in the direction of the electric field vector of the wave propagating in the waveguide and in parallel with the axis of the waveguide.

It is known to heat thin strips of material having dielectric losses, for example, of textile, paper or synthetic material, in the field of an electromagnetic wave as they pass continuously through a waveguide. During this process, the strip of material is pulled through a double slot in the Wall of the waveguide which is formed in the neutral surface of the waveguide in the direction of the electric field vector of the wave and in parallel with the axis of the waveguide. The electric field vector of the wave produced within the waveguide is situated in the plane of the strip of material so that a maximum amount of field energy is extracted from the wave in the form of heat.

Such a device may also be designed as a waveguide system folded once or several times, in which the straight sections of the waveguide have slots through which the thin strip of material is pulled. The strip of material extracts energy from the field in each section of the waveguide, resulting in a comparatively high attenuation of the wave propagating in the waveguide and hence more rapid heating.

Such high-frequency heating devices comprising waveguides are often used to dry moist strips of material or to evaporate solvents, for example, of lacquers, colouring matter or adhesives, or harden thin layers of such substances, for example, melamine-resin colouring matter or suspension colouring matter. A particular difficulty arises in that a standing wave occurs due to reflections of the wave which is propagating in the waveguide. Such reflections may originate from the material to be treated, but may also occur, for example, at the bend connecting two sections of the waveguide. Further, in case of substances having low dielectric losses, it is in many cases necessary to produce an additional reflection at the end of the waveguide in order to obtain an increase of the field and hence an increase in the conversion of energy.

3,413,433 Patented Nov. 26, 1968 ice The layer is heated unevenly due to the nonuniformity of the field-strength distribution in the direction of propagation of the wave. This is very disturbing in processes which greatly depend upon temperature, especially if uniform heating is to be obtained. There are processes in which the requirement of uniform heating cannot be technically fulfilled with known slotted waveguides.

This is the case, for example, with polymerisation processes or other conversions which are accomplished within a temperature range of, for example, 1%. To adhere to this range, the power fluctuations would have to be less than 1%, that is to say in no section of the waveguide would the standing wave ratio s: U,,,,,,,/ U be allowed to exceed the value 1.005, a value which is hardly obtainable with high class matched transmission lines.

Several efforts have been made to obtain a uniform distribution of the energy in a high-frequency heating device of the kind referred to. Thus it has previously been suggested, for example, to bring about a variation in the oscillation modes in the waveguide by providing a reflective element at the end of the waveguide, for example, a rotatable metallic body or a reflective wall moved periodically in the longitudinal direction of the waveguide, which varies the reflection coeflicient. However, this does not yield the result desired, since such a variation takes place, at least in part, in steps, so that discrete regions of different field strength still exist over the length of the waveguide, which means that, when reckoned over a long period, the energy distribution is not completely uniform throughout the volume of the material to be treated. Furthermore, during the movement of such reflective elements, in addition to the stepped variation of the oscillation modes, not only the phase but also the amount of reflected energy varies so that, in addition to the phase, the amplitudes in the maxima and minima of the standing wave vary.

Such movable reflective elements therefore do not yield the desired result or only very incompletely.

Furthermore, in the case of a folded wave-guide system, the reflections at the bends cannot be influenced by a single reflective element at the end of the waveguide so that in this system each bend would have to be provided with a reflective element. Such a plurality of reflective elements would, apart from the above mentioned disadvantages, be complicated and expensive.

Acording to another suggestion, the microwave generator feeding the waveguide system is modulated in frequency so that, due to the variation in the wavelength of the energy supplied, the wavelength in the waveguide also varies and the standing wave in the wavegmide shifts. However, frequency-modulated microwave generators of high power are complicated and expensive. Furthermore, it is very diflicult to make the input impedance of the waveguide system constant over a comparatively large range of frequencies and minimize the reaction of load variations to the generator.

An object of the invention is to obviate the disadvantages above described. According to the invention, the problem of obtaining a continuous shift, corresponding to the desired energy distribution of the standing Wave which occurs in the waveguide even with optimum matching, is solved by connecting movable means to the waveguide which continuously vary the wave-length at least in the'sections provided with longitudinal slots. By a suitable choice of the positions of the movable means as a function of time, it may be achieved that, during the movement of said means, the time integral of the power is the same for any point of the material to be treated. However, by a suitable choice of the wavelength and the dimensions of the Waveguide, it is also possible to obtain a desired energy distribution of another kind by giving the movable means, as a function of time, corresponding locations.

In one advantageous embodiment of the invention, the cross-sectional dimension of the waveguide normal to the plane of the slots is variable since at least the slotted sections of the waveguide are relatively movable in this direction. It is preferable that the slot extends throughout the length of the waveguide. This is important, especially in a folded waveguide system comprising a plurality of sections, since the continuous slot avoids the necessity of providing matched transition pieces, movable contact devices or the like at each bend connecting the waveguide sections.

Several advantages are obtained by using a folded waveguide system comprising a plurality of sections having a continuous slot, the ends of the slotted section being rigidly connected together and to the connecting parts of the high-frequency heating device. The movement does not cause difiiculty in mechanical respect since the sections of a folded waveguide system having a continuous slot are flexible enough. Contact devices or other transition elements to the fixed parts of the high-frequency heating device are avoided, however, and the reaction of the moved system to the generator is negligible. This may be explained by observing that the impedance transformation which continuously takes place in one direction from the end of the waveguide remote from the generator to the centre, continuously takes place with the same values in the opposite direction from the centre to the end of the Waveguide adjacent the generator, so that the two parts neutralize each others influence from one end to the other.

The wavelength in the waveguide may also be varied continuously by movably arranging a field-displacing body which continuously varies the wavelength in the waveguide, at least in one half of the waveguide at least in the region of the slotted parts thereof. The field-displacin g body may be made from a material having a relative dielectric constant greater than unity, but may alternatively be metallic. Said field-displacing body preferably is a ledge-shaped or rod-shaped body which can slide in a direction at right angles to the plane of the slots and which fills part of one half of the waveguide.

In a waveguide system comprising at least two folded waveguide sections, the displacing body may be arranged throughout the length of the waveguide including the bends. However, it is also possible to arrange the field- 'displacing body only in the straight sections which are slotted over the greater part of their length and, in order to avoid reaction, to give the bend an electrical length of n-times the half mean wavelength in the waveguide.

To obtain a great variation of the wavelength in the waveguide for a small movement of said means varying the wavelength, it is advantageous that the movement of the means takes place in the region of the greatest steepness of the function showing the relationship between the wavelength in the waveguide and the location of the means.

The mechanical resonance of the system comprising the movable means for varying the wavelength in the waveguide and the driving elements thereof may advantageously be utilized for reducing the driving energy.

In order that the invention may be readily carried into effect, it will now be described in detail, by way of example, with reference to the accompanying diagrammatic drawings, in which:

FIGURE 1 shows a high-frequency heating device for thin dielectric strips of material, comprising a folded waveguide system having a continuous slot and the two parts of which are movable with respect to one another;

FIGURE 2 is a sectional view of one waveguide section of the device of FIGURE 1, at right angles to the axis of the waveguide;

FIGURE 3 is a graph showing the relationship be- 4 tween the wavelength in the waveguide and the crosssectional dimension of the waveguide, and

FIGURE 4 is a sectional view, at right angles to the axis of the waveguide, of a waveguide section of another embodiment of the device according to the invention in which the wavelength in the waveguide may be varied by moving a field-displacing body at right angles to the plane of the slots.

In the device of FIGURE 1, the high frequency energy is coupled at the position indicated by an arrow 1, from a high-frequency generator (not shown) of fixed frequency through a connecting piece 2 into a folded rectangular waveguide system 3. The connecting piece 2 and the folded waveguide system 3 are connected together by means of flanges 4. A terminating impedance 6, the function of which will be referred to hereinafter, is connected to the end of the waveguide system 3 by means of flanges 5.

By means of continuous slots 7 formed in the middle of the two broad walls of the waveguide, the waveguide system 3 is divided into two identical parts 8 and 9 (see FIGURE 2) which are rigidly connected together at their ends by means of the flanges 4 and 5.

From FIGURE 2 it may be seen that the waveguide halves 8 and '9 are closed by means of dielectric foils 10 which prevent the gases, vapours and the like which emerge during heating, from entering the waveguide and giving rise in situ to disturbance of the heating process, for example, due to condensation.

A strip of material 11 to be heated may be pulled through the waveguide system 3 by means of a conveyor device, not shown, during Which movement the high-frequency energy is converted into heat due to the dielectric losses in the strip of material or in any layer present thereon, for example, a colour layer, in heating spaces 12 (FIGURE 2) in the plane of the slots.

A driving rod 15 is secured to the two central sections of the waveguide through a plate 13 and a hinge 14. The driving rod supports a follower 16 which is pushed by a spring 17 against a driving cam 18. The cam may be driven in an arbitrary direction of rotation, for example, that indicated by an arrow 20, by means of a shaft 19. During the rotation of the shaft 19, the driving rod 15 performs a vertical movement which is transferred to the upper part 8 of the waveguide system 3, which is thus moved periodically in the direction of the arrow 21 at right angles to the plane of the slots in accordance with a time function given by the curvature of the cam 18. Due to the flexibility of the wave guide system, the adjacent sections of the waveguide follow this movement up to the fixed points of the flanges 4 and 5 depending on the flexibility.

The graph of FIGURE 3 shows the relationship between the wavelength in the waveguide and the mutual position of the two parts 8 and 9 of the waveguide. The dimension a of the broad walls of the waveguide (FIG. 2) is plotted on the axis of abscissae (FIGURE 3) and the associated wavelength A in the waveguide is plotted on the axis of ordinates.

The curve shown in FIGURE 3 results from the formula:

where x, is the free-space wavelength and A is the cutoff wavelength of the waveguide for given dimensions.

In the present rectangular waveguide A =2a. That is to say, the dimension b has no influence on the cut-off wavelength and, according to the above-mentioned formula, it has no influence on the wavelength A in the waveguide.

It is preferable to lay the stroke of the relative movement of the waveguide parts in the steep portion of the curve of FIGURE 3. In this case, a great variation of 3 the wavelength in the waveguide is obtained for a small stroke.

Measurements have demonstrated that, with a suitable choice of the working point on the curve, a stroke of several millimetres already sufiices to obtain a uniform energy distribution. It has been found that a total distance of approximately mm. between the adjacent inner sides of the waveguide parts 8 and 9 at a Wavelength of 12 cm. is permissible without an undue amount of high-frequency energy radiating through the slots.

The relationship between the energy distribution over the width of material to be heated and the mutual position of the Waveguide parts (dimension a) may be obtained in the simplest way by experiment. In accordance with the result thereof, the shape of the driving cam can be determined. An accurate calculation of this relationship is diflicult since the energy distribution depends upon the local field-strength, which in turn depends not only upon A but also upon other unsurveyable parameters, for example, the variation of the characteristic impedance of the waveguide upon variation in the cross-sectional dimensions.

The terminal impedance 6 absorbs the power of the generator if the installation is used without passage of material, or if the material passing through has lower dielectric losses due to the heating. The length of the waveguide may be chosen in view of the attenuation by the material to be treated, so that part of the load on the generator is always formed by the terminating impedance which thus absorbs as a basic load, for example, approximately one fifth to one tenth of the input power, thus achieving that the standing waves already for this reason become weaker and possibly the reaction of the relative movement of the waveguide parts to the generator is reduced as a result of the greater attenuation.

The outer edges of the strip of material are at a distance of several half wavelengths from the bends of the waveguide system which assist in producing the reflections. Since a sufficient shift of the maxima and minima occurs only at a given distance from the reflective surfaces due to the addition of the shifts of the individual parts of the wave, a very uniform energy distribution will be achieved only in situ.

FIGURE 4 is a cross-sectional view of a waveguide in which the wavelength may be varied by two field-displacing bodies 22. These bodies can be moved symmetrically within the parts of the waveguide, in the directions indicated by arrows 24, by means of driving rods 23 of insulating material. The field displacing bodies 22 may consist of either a metal or a dielectric having a dielectric constant greater than unity, depending on the desired variation of the wavelength in the waveguide.

In the position in which the field-displacing bodies are in the middle of the waveguide in the region of maximum field strength, they increase the capacity of the waveguide and hence the electrically active length of the broad walls of the waveguide, resulting in a similar effect to that obtained by the mechanical increase of the dimension :1 in FIGURES 2 and 3.

It is, of course, also possible to use a single field-displacing body if a smaller variation of the wavelength in the waveguide is desired.

The relationship between the wavelength in the waveguide and the position of the field-displacing body is determined inter alia by the shape of the cross-sectional area of the field-displacing body. Thus, if this cross-sectional area has a suitable shape, a sinusoidal time function of the position of the body may be standing waves already for this reason become weaker and possibly the reaction of the relative movement of the waveguide parts to the generator is reduced as a result of the greater attenuation.

The outer edges of the width of material are at a distance of several half wave-lengths from the bends of the waveguide system which assist in producing the re- 6 flections. Since a sulficient shift of the maxima and minima occurs only at a given distance from the reflective surfaces due to the addition of the shifts of the individual parts of the wave, a very uniform energy distribution will be achieved only in situ.

FIGURE 4 is a cross-sectional view of a waveguide in which the wave-length may be varied by two field-displacing bodies 22 which can be moved symmetrically within the parts of the waveguide in the directions indicated by arrows 24 by means of driving rods 23 of insulating material. The field displacing bodies 22 may consist of either a metal or a dielectric having a dielectric constant greater than unity depending on the desired variation of the wavelength in the waveguide.

In the position in which the field-displacing bodies are in the middle of the waveguide in the region of maximum field strength, they increase the capacity of the waveguide and hence the electrically active length of the broad walls of the waveguide, resulting in a similar effect as obtained by the mechanical increase of the dimension a in FIG- URES 2 and 3.

It is, of course, also possible to use a single field-displacing body if a smaller variation of the wave-length in the waveguide is desired.

The relationship between the wave-length in the waveguide and the position of the field-displacing body is determined inter alia by the shape of the cross-sectional area of the field-displacing body. Thus, if this cross-sectional area has a suitable shape, a sinusoidal time function of the position of the body may be obtained with a uniform en ergy distribution. In this way it is possible to utilize very advantageously the mechanical resonance of the system.

We claim:

1. A high-frequency heating device for heating thin strips of material, comprising a waveguide provided at its periphery with two longitudinal slots extending over at least part of its length, a source of high frequency energy coupled to said waveguide, the strip of material being arranged in the plane connecting said slots which extends in the direction of the electrical field vector of the wave propagating in the waveguide and in parallel with the axis of the Waveguide, and movable means connected to the waveguide which continuously vary the cut-off wavelength of the waveguide during the heating period at least in the parts of the waveguide which are provided with said longitudinal slots.

2. A high-frequency heating device as claimed in claim 1 wherein the parts of the waveguide provided with slots are relatively movable in a direction normal to the plane of the slots thereby to vary the cross-sectional dimension of the waveguide normal to the plane of the slots.

3. A high-frequency heating device as claimed in claim 2 wherein said waveguide is provided with a continuous slot throughout its length.

4. A high-frequency heating device as claimed in claim 3 wherein said waveguide comprises a plurality of waveguide sections in the shape of a folded waveguide having a continuous slot, and means rigidly connecting the ends of the slotted parts together and to the connection parts of the high-frequency heating device.

5. A high-frequency heating device as claimed in claim 1 wherein said movable means comprises a movable fielddisplacing body arranged in at least one half of the wave guide in the region of the slotted parts, and means for moving said body during the heating period thereby to continuously vary the wavelength in the waveguide.

6. A high-frequency heating device as claimed in claim 5 wherein said field-displacing body is made from a material having a dielectric constant greater than unity.

7. A high-frequency heating device as claimed in claim 5 wherein said field-displacing body is composed of a metallic material.

8. A high-frequency heating device as claimed in claim 7 wherein the field-displacing body is a ledge-shaped or rod-shaped body which can move in a direction normal 7 to the plane of the slots and fills part of one'half of the waveguide.

9. A high-frequency heating device as claimed in claim wherein said waveguide comprises at least two interconnected waveguide sections in the shape of a folded waveguide, the length of each bend connecting adjacent sections of the waveguide being chosen to have an electrical length of nxx/2, wherein n is a whole number and A is the wavelength.

10. A high-frequency heating device as claimed in claim 1 wherein the location of the means varying the wavelength in the waveguide is chosen so that the movement thereof as a function of time is such that the time integral of the power is the same for any point of the material to be treated.

11. A high-frequency heating device as claimed in claim 2 wherein the movement of the means varying the wavelength in the waveguide takes place in the region of the maximum steepness of the function representing the relationship between the wavelength in the waveguide and the location of said means.

12. A high-frequency heating device as claimed in claim 5 wherein the system comprising the movable means for varying the wavelength in the waveguide and the body moving means is in mechanical resonance.

13. A high-frequency heating device as claimed in claim 5 wherein the cross-sectional area of the displacing body is shaped so that the time function of the position of the field-displacing body is sinusoidal if the energy distribution is uniform.

14. High-frequency heating apparatus for strip dielectric material comprising, a folded waveguide in the path of said strip material and arranged to define a serpentine path for high-frequency energy, a source of highfrequency energy coupled to said waveguide near one end thereof to propagate wave energy through said serpentine path to the other end of said waveguide, a plurality of aligned slots extending longitudinally along the walls of said waveguide to define a plane channel for the passage of said strip material that extends parallel to the direction of the electric field vector of said wave energy propagating in the waveguide, and means located intermediate said one end and said other end of said waveguide for effectively varying a cross sectional dimension of the waveguide during the heating period thereby to continuously vary the wave length in the waveguide.

15. Apparatus as described in claim 14 wherein said means for varying comprises means coupled to said waveguide for relatively moving the slotted parts of the waveguide toward and away from one another in a direction normal to the plane of the slots.

16. Apparatus as described in claim 14 wherein said waveguide has a rectangular cross section and includes a continuous slot in the side walls thereof extending along the length of said serpentine path and defining said channel, said means for varying comprising, an element coupled to a wall normal to said side walls and movable with a reciprocating motion normal to the plane of said channel, and means for reciprocally moving said element thereby to physically vary a cross sectional dimension of said waveguide continuously during a heating period.

17. Apparatus as described in claim 14 wherein said means for varying comprises, first and second movable members mounted within said waveguide on opposite sides of said channel, and means for reciprocally moving said first and second members in a direction normal to the plane of the slots.

References Cited UNITED STATES PATENTS 2,560,903 7/1951 Stiefel 219-10.61 X 2,909,635 10/1959 Haagensen 2l910.55 3,177,333 4/1965 Lamb 219-10.55 3,304,399 2/1967 Timmermans et al. 219-1061 X FOREIGN PATENTS 1,264,758 5/1961 France.

OTHER REFERENCES Bosch, German Application 1,186,570, printed Feb. 4, 1965.

RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,413,435 Dated November 26, 1968 Inventor(s) Franciscus Timmermans et al.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5, line 68, cancel "standing waves";

Column 5 lines 69-75 cancel lines;

Column 6, lines l-29 cancel lines Column 6 line 30, cancel "position of the body may be".

"his certificate supersedes Certificate of Correction issued May 9, 1970.

Signed and sealed this 14th day of July 1970.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents 

